1. Field of the Invention
This invention relates to an improved method for performing cryosurgery and, more particularly, to a mask or drape having a silicone gel surface which adheres to the skin at low temperatures.
2. Prior Art
Cryotherapy is a well established procedure for removing unwanted tissue (target tissue) from the skin. The advantages of treating target tissue such as a tumor with cryosurgery include a higher cure rate and reduced cost per procedure. Small tumors take less than 20 minutes to freeze while larger cancers (i.e. breast and vulva) can take up to 3 hours. In addition, healthy skin can tolerate temperatures down to -12.degree. centigrade. Problems occur when the temperature gets below -20.degree. C. In cryosurgery, an area of the skin incorporating the unwanted or diseased tissue is frozen to reduce the temperature of the tissue to a lethal temperature causing cell death in the diseased tissue which is subsequently rejected by the body. A safety margin consisting of normal tissue adjacent to and surrounding the diseased tissue is also frozen to prevent any diseased tissue cells from remaining behind.
When performing cryosurgery it is desirable to "freeze quickly and thaw slowly." In accordance with this dictum, liquid nitrogen is a preferred coolant and is widely employed for freezing unwanted target tissue. Liquid nitrogen (LN) is relatively inexpensive and easy to handle. The disadvantage of using liquid nitrogen is that when LN is applied to the surface of the skin it freezes the outermost layer before cooling the target tissue to a depth sufficient to kill all diseased cells. The frozen layer of skin forms a barrier to heat conduction causing additional LN applied to the target tissue to run off the frozen surface surrounding nontarget tissue before it can evaporate. To prevent such damage to nontarget tissue, practitioners use intermittent spraying of coolant onto the target tissue. This enables the heat to be conducted from the deeper tissue to the outer layer where the caloric content is expended vaporizing the liquid nitrogen and equilibrating the tissue temperature to a desired depth. Thus, in order to prevent inadvertent burning of nontarget tissue, an intermittent spray is employed.
Silicone sheeting has been used for many years as a wound dressing. There are other medical conditions, such as burns, which benefit by the application of silicone sheeting thereto. In some instances, the silicone sheeting is permeable to liquids so that a suppurating wound may remain dry. In other instances, the sheeting is permeable to gases. Unfortunately, the currently available sheeting used for wound dressing is inoperable as a mask for cryosurgery because either the sheeting is permeable to gas and/or liquids or the surface of the sheeting (which contacts the skin) comprises a gel which loses its ability to adhere to the skin at temperatures below -20.degree. C. As a consequence, the material used in commercially available silicone gel type wound dressings, such as "Epi-Derm.RTM.", does not provide a suitable masking material for enabling the practitioner to use a constant stream of liquid nitrogen to freeze unwanted tissue without injury to non-target tissue.